Physics of Dust and Sand Storms

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Description on how dust storms are generated and what are the proper means to detect it from space.

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    Physics of Dust and Sand Storms

    Alee K. Obeid

    BSc. Mechanical Engineering (Aeronautics) High Level Post Graduate Aerospace Engineering Student

    1. Introduction The wind-driven emission, transport, and deposition of sand and dust by wind are termed

    Aeolian processes, after the Greek god Aeolus, the keeper of the winds. Aeolian processes occur

    wherever there is a supply of granular material and atmospheric winds of sufficient strength to

    move them. On Earth, this occurs mainly in deserts, on beaches, and in other sparsely vegetated

    areas, such as dry lake beds. The blowing of sand and dust in these regions helps shape the

    surface through the formation of sand dunes and ripples, the erosion of rocks, and the creation

    and transport of soil particles. Moreover, airborne dust particles can be transported thousands of

    kilometers from their source region, thereby affecting weather and climate, ecosystem

    productivity, the hydrological cycle, and various other components of the Earth system.[Ref.(1)].

    But Aeolian processes are not confined to Earth, and also occur on Mars, Venus, and the

    Saturnian moon Titan (Greeley and Iversen 1985). On Mars, dust storms occasionally obscure the

    sun over entire regions of the planet for days at a time, while their smaller cousins, dust devils,

    punctuate the mostly clear daytime skies elsewhere (Balme and Greeley 2006). The surface of

    Mars also hosts extensive fields of barchans, transverse, longitudinal, and star-like dunes, as well

    as other exotic dune shapes that have not been documented on Earth (Bourke et al. 2010). On

    Venus, transverse dunes have been identified by the Magellan orbiter (Weitz et al. 1994), while

    the Cassini orbiter has documented extensive longitudinal sand dunes on Titan (Lorenz et al.

    2006). [Ref. (1)].

    The terms dust and sand usually refer to solid inorganic particles that are derived from

    the weathering of rocks. In the geological sciences, sand is defined as mineral (i.e., rock-derived)

    particles with diameters between 62.5 and 2,000 m, whereas dust is defined as particles with

    diameters smaller than 62.5 m (note that the boundary of 62.5 m differs somewhat between

    particle size classification schemes, see Shao 2008, p. 119). In the atmospheric sciences, dust is

    usually defined as the material that can be readily suspended by wind (e.g., Shao 2008), whereas

    sand is rarely suspended and can thus form sand dunes and ripples, which are collectively termed

    bed forms. [Ref. (1)].

    Dust storms are among the most severe environmental problems in certain regions of the

    World. In where they occur most of the dust in the atmosphere is from Aeolian origin. Estimates

    of the total Aeolian dust from deserts in the atmosphere are about ton/yr (Ning Ai and Karen R. Polenske). Several authors (JungeC 1979): Ganor E, Mamane Y 1982: Morales C 1979)

    have estimated that the Sahara desert alone contributes ton/yr or between 4066% of the total dust. Dust storms may be traced as far as 4000 km from their origin. [Ref. (2)].

    Dust storms may cause a variety of problems. One of the major problems is a

    considerable reduction of visibility that limits various activities, increases traffic accidents, and

    may increase the occurrence of vertigo in aircraft pilots (Morales C., 1979; Hagen L.J, Woodruff

    N.P. 1973; Middleton N.J, Chaudhary QZ 1988; Dayan U, Heffter J, Miller J, Gutman G 1991;

    Yong-Seung C, 1996). Other environmental impacts, reported in the literature (Hagen L.J,

    Woodruff N.P. 1973; Mitchell J.M. 1971; Fryrear D.W. 1981; Victor R. Squires. 2007; Jauregui

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    E. 1989; Liu C.M. Ou S.S. 1990; Yong-Seung C, Ma-Beong Y. 1996 and Ning Ai and Karen R.

    Polenske) include reduced soil fertility and damage to crops, a reduction of solar radiation and in

    consequence the efficiency of solar devices, damage to telecommunications and mechanical

    systems, dirt, air pollution, increase of respiratory diseases and so on. Three main categories of

    reduced visibility are often used to describe the severity of dust storms.

    Blowing Dust the horizontal visibility (due to Aeolian dust), is less than 11 km. Dust Storm the horizontal visibility is less than 1000 m. Severe Dust Storm the horizontal visibility is less than 200 m. [Ref. (2)].

    In some seasons in certain regions of the Middle East and North Africa and for about

    30% of the time on average, the dust conditions in the lower troposphere fall into one of these

    three categories. Thus, in these regions, dust storms are a very frequent phenomenon and a better

    knowledge of their spatial and temporal distribution is of prime importance. A positive

    correlation exists between the quantity of dust in the air, and the wind velocity. Whereas, a

    negative correlation exists between dust amount and the particles size. Precipitation and/or vegetation coverage may reduce considerably the amount of dust in the air for a given wind

    velocity and/or particles size (Bagnold R.A. 1941; Gillette D.A. 1979; Mitchell J.M. 1971). Thus, a study of the atmospheric circulation and its impact on the precipitation regime in a given

    region is crucial to understand the dust distribution in that region. [Ref. (2)].

    A previous attempt to delimit the regions in the Middle-East according to the seasons of

    main activity was done by Middleton (Middleton N.J. 1988). He analyzed the dust distribution

    over Syria, Lebanon, Jordan, Israel, Saudi Arabia, Yemen, Iraq and Iran and his analyses were

    based on short periods of data recording and on other data collected over varying lengths of time

    from the 1950s and 1960s. The present study extends Middletons analysis in three ways: (a) the study area was extended to include also data from Turkey, Cyprus, Egypt and Sudan, (b) the

    analysis period was extended to 21 years (19731993) and (c) the clustering of the different stations into coherent regions was done in an objective way using cluster analysis. [Ref. (2)].

    1.1 Manners of Wind-Blown particle transport

    The transport of particles by wind can occur in several manners, which depend

    predominantly on particle size and wind speed (Figure 1.1). As wind speed increases, sand

    particles of ~100 m diameter are the first to be moved by fluid drag. After lifting, these particles

    hop along the surface in a process known as saltation (Bagnold 1941, Shao 2008), from the Latin

    salto, which means to leap or spring. The impact of these saltators on the soil surface can

    mobilize particles of a wide range of sizes. Indeed, dust particles are not normally directly lifted

    by wind because their interparticle cohesive forces are large compared to aerodynamic forces.

    Instead, these small particles are predominantly ejected from the soil by the impacts of saltating

    particles (Gillette et al. 1974, Shao et al. 1993a). Following ejection, dust particles are susceptible

    to turbulent fluctuations and thus usually enter short-term (~ 20 - 70 m diameter) or long-term

    (< ~20 m diameter) suspension (Figure 1.1). Long-term suspended dust can remain in the

    atmosphere up to several weeks and can thus be transported thousands of kilometers from source

    regions (Gillette and Walker 1977, Zender et al. 2003a, Miller et al. 2006). These dust aerosols

    affect the Earth and Mars systems through a wide variety of interactions. [Ref.(1)].

    The impacts of saltating particles can also mobilize larger particles. However, the

    acceleration of particles with diameters in excess of ~500 m is strongly limited by their large

    inertia, and these particles generally do not saltate (Shao, 2008). Instead, they usually settle back

    to the soil after a short hop of generally less than a centimeter, in a manner of transport known as

    reptation (Ungar and Haff 1987). Alternatively, larger particles can roll or slide along the surface,

    driven by impacts of saltating particles and wind drag forces in a mode of transport known as

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    creep (Bagnold 1937). Creep and reptation can account for a substantial fraction of the total wind-

    blown sand flux (Bagnold 1937, Namikas 2003). [Ref.(1)].

    The transport of soil particles by wind can thus be crudely separated into several physical

    regimes: long-term suspension (< ~20 m diameter), short-term suspension (~20 70 m), saltation (~70 500 m), and reptation and creep (> ~500 m) (Figure 1.1). Note that these four transport modes are not discrete: each mode morphs continuously into the next with changing

    wind speed, particle size, and soil size distribution. The divisions based on particle size between

    these regimes are thus merely approximate. [Ref.(1)].

    Figure 1.1. Schematic of the different modes of Aeolian transport. [Ref.(1)].

    A recent study finds that the initial saltation of sand particles induces a static electric field

    by friction. Saltating sand acquires a negative charge relative to the ground which in turn loosens

    more sand particles which then begin saltating. This process has been found to double the number

    of particles predicted by previous theories. (Electric Sand Findings 2008). [Ref. (2)].

    1.2 Contribution of study of wind-blown sand and dust to the Earth and planetary sciences

    Wind-blown sand has shaped a substantial portion of the Earths surface by creating sand dunes and ripples in both coastal and arid regions (Bagnold 1941, Pye and Tsoar 1990), and by

    weathering rocks (Greeley and Iversen 1985), which contributes to the creation of soils over long

    time periods (Pye 1987). Since Aeolian processes arise from the interaction of wind with the

    surface, the study of aeolian bedforms (such as dunes) and aeolian sediments (such as loess soils

    or Aeolian marine sediments) can provide information on the past state of both the atmosphere

    and the surface (Greeley and Iversen 1985, Pye and Tsoar 1990, Rea 1994). For instance,

    important constraints on both the ancient and contemporary history of Mars are provided by the

    inference of formative winds and climate from the morphology and observed time evolution of

    aeolian surface features (Greeley et al. 1992a). [Ref. (1)].

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    2. The physics of wind-blown sand (saltation) Saltation plays a central role in Aeolian processes since it usually initiates the other forms

    of transport, including the emission of dust aerosols that subsequently travel in suspension.

    Saltation is initiated when the wind stress is sufficient to lift surface particles into the fluid stream, which for loose sand occurs around ~ 0.05 N/m2 (Greeley and Iversen 1985). Following initiation, the lifted particles are accelerated by wind into ballistic trajectories and the

    resulting impacts on the soil bed can eject, or splash, new saltating particles into the fluid stream.

    This process produces an exponential increase in the particle concentration (Durn et al. 2011a),

    which leads to increasing drag on the wind, thereby retarding the wind speed in the saltation layer

    (Bagnold 1936). It is this slowing of the wind that acts as a negative feedback by reducing

    particle speeds, and thus the splashing of new particles into saltation, which ultimately limits the

    number of saltating particles (Owen 1964) and thereby partially determines the characteristics of

    steady state saltation. [Ref. (1)].

    The physics of Aeolian saltation can thus be roughly divided into four main physical

    processes (Anderson and Haff 1991, Kok and Renno 2009a): (i) the initiation of saltation by the

    aerodynamic lifting of surface particles, (ii) the subsequent trajectories of saltating particles, (iii)

    the splashing of surface particles into saltation by impacting saltators, and (iv) the modification of

    the wind profile by the drag of saltating particles. [Ref. (1)].

    2.1 Physical Processes of Aeolian Saltation

    2.1.1 Emergence of Saltation: The Fluid Threshold

    Saltation is initiated by the lifting of a small number of particles by wind stress (Greeley and

    Iversen 1985). The value of the wind stress at which this occurs is termed the fluid or static

    threshold (Bagnold 1941). This threshold depends not only on the properties of the fluid, but also

    on the gravitational and interparticle cohesion forces that oppose the fluid lifting. Dust or Sand

    Storms can be initiated by three main ways (a) Aerodynamic Entrainment, (b) Soil aggregate

    disintegration, and (c) Saltating aggregate disintegration Figure 2.1. A schematic of the resulting

    force balance on a surface particle subjected to wind stress is presented in Figure 2.2. [Ref. (1)].

    Figure 2.1. General Scheme showing three different ways to initiate Dust or Sand Storms

    (a) Aerodynamic Entrainment, (b) Soil aggregate disintegration, and (c) Saltating aggregate

    disintegration.

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    The fluid threshold is distinct from the dynamic or impact threshold, which is the lowest

    wind stress at which saltation can be sustained after it has been initiated. For most conditions on

    Earth and Mars, the impact threshold is smaller than the fluid threshold because the transfer of

    momentum to the soil bed through particle impacts is more efficient than through fluid drag.

    An expression for the fluid threshold can be derived from the force balance of a

    stationary surface particle (Figure 2.1). The surface particle will be entrained by the flow when it

    pivots around the point of contact with its supporting neighbor (P in Figure 2.1). This occurs

    when the moment of the aerodynamic drag (Fd) and lift (Fl) forces barely exceeds the moment of

    the interparticle (Fip) and gravitational (Fg) forces (Greeley and Iversen 1985, Shao and Lu

    2000). At the instant of lifting, we thus have that:

    ( )

    Where , , and are the moment arms in Figure 2.1, which are proportional to the particle

    diameter . The effective gravitational force in a fluid, which includes the buoyancy force, equals

    ( )

    Where g is the constant of gravitational acceleration and Dp is the diameter of a sphere with the

    same volume as the irregularly-shaped sand particle. The particle density p depends on the composition of the sand, but equals approximately 2650 kg/m3 for quartz sand on Earth.

    Furthermore, the drag force exerted by the fluid on a surface particle protruding into the flow is

    given by (Greeley and Iversen 1985, Shao 2008) [Ref. (1)].

    Where is the air density, is a dimensionless coefficient of the order of ~10 and the shear

    velocity is a scaling parameter proportional to the velocity gradient in boundary layer flow and is defined as (Stull 1988, White 2006) [Ref. (1)].

    The fluid shear stress is equivalent to the flux of horizontal momentum transported downward through the fluid by viscous and turbulent mixing. A straightforward expression for the fluid

    threshold shear velocity at which saltation is initiated can now be obtained by combining Eqs. (2.1) - (2.3), w...